I an mew to this and need help.

While going through data sheets of flow meters (Venturi Meter, Orifice, Wedge flow meter) issued to different vendors, I observed the following..

A)Design pressure @Min & Max flow is given for Venturi whereas only max values in case of the Orifice and Wedge Meter.

B)Flow rate and Inlet Temp/Pressure @(Min/Normal/Max flow) is mentioned for Venturi and Wedge flow meter whereas for Orifice, Flow rate and Inlet Temp/Pressure @ (Normal/Max flow only) is mentioned.

C)Viscosity @ Normal flow is mentioned in Venturi & Orifice whereas for Wedge meter, Viscosity@ (Min/Normal/Max flow) is mentioned.

D)Vapor Pressure is not mentioned in Venturi & Orifice whereas for Wedge meter, Vapor pressure @ Min/Normal/Max flow is mentioned.

E)Critical pressure is mentioned for Venturi and Wedge flow whereas it is not mentioned for Orifice...

Please clarify the importance of these data

 

This is rather a general question and there is a lot that might be said.

The best way to get some insights is to look up the vendor data sheets for the various technologies and see what data they specify under performance data and process limits.

Most important of all is to determine how a particular sensor works and then see what its vulnerabilities and advantages are.

Both should be available via an internet search.

Most of these technologies have a primary element which generates a pressure drop as a function of the volumetric flow rate or velocity.

You also have to sort out which are being discussed for liquid flow and which for gas.
Viscosity, for example, may be very critical for liquid flows and not mentioned for gas flows.

Some sensors are specified based on the selected secondary element for the meter and some are determined based on the primary element sizing.

For example, a single orifice plate may be capable of several different flow rate ranges depending which dP transmitter (and dP span) is chosen. Indeed, in some applications multiple dP transmitters can be used with a single primary element. This will affect what data is asked for or given.

 

While going through data sheets of flow meters (Venturi Meter, Orifice, Wedge flow meter) issued to different vendors, I observed the following..

Design Pressure and Design Temperature are conditions used to specify the mechanical design envelope for the instrument. These do not relate to the measurement being made at all. These two terms are most relevant for all Flanged Inline instruments and valves. Examples being Vortex, Coriolis, ultrasound flowmeters, Level Gauges, Thermowells, Control valves, OnOff valves etc.

Using the Mechanical Design conditions, the engineer should verify that the selected instrument is suitable Mechanically for the application.

This step is important because very often, the piping and vessels are carbon steel, but the instruments are stainless steel. Stainless Steel flanges have a LOWER pressure rating for the same flange class as compared to carbon steel flanges. Almost every project, I come across cases where the flange of an instrument needs to be uprated to suit the piping design conditions.

Orifice plates are usually fitted between flanges of the same material and rating as the rest of the piping system or a minimum of Class 300. Hence they do not require the mechanical design conditions specified each time. Unless it is a flanged meter run you are specifying or flanged orifice carriers.

Some meters like the wedge, or vcone get supplied often as a flanged or weld in spool made from the same material as the pipe. In this case, some engineers may not specify the design conditions.
For the actual measurement conditions, my preference is to specify a full set of data for each flow case. i.e. inlet pressure, inlet temperature, density (mol weight, compressibility, cp/cv for gases), viscosity etc.

In many cases, the properties of the fluid (i.e. density, mol weight etc) may not change for the different flow cases. However in the most general case, these properties may change. As an example, in a case where the properties of the fluid get affected by the efficiency of a catalyst in the upstream equipment, there can be a significant variation in the density of the fluid. In case of a DP flowmeter, if the control system does not compensate for the changed properties, of the fluid, the operators may see the wrong flow rate on the HMI and their response will be in appropriate.

For orifice plates and other fixed geometry DP flow meters, I usually specify one set of properties as a sizing case. The other cases I use to work out the measured flow rates for the different set of operating conditions.

It is assumed (often incorrectly) that for an orifice plate in liquid service vapour pressure is not required to be known. Now the vapour pressure is not required for sizing an orifice plate, but it is necessary to verify that there will be no cavitation or flashing. I have not seen many orifice datasheets with a field for vapour pressure. A case where this usually becomes a problem is in distillation column bottoms flow measurement. The fluid sits just under its boiling point and the slightest pressure drop can cause cavitation. If you don’t ask for the information, how can you check for the problems?

I need to read a bit more into the critical pressure and critical temperature data but yes it is sometimes required for Orifice plates as well.

Hope this helps and has not been too boring.

Regards
Sumeet